Personalization fee with lineUpdate

api_version = "2026-10"

[[extensions]]
name = "Personalization fee with lineUpdate"
handle = "personalization-fee-line-update"
type = "function"

  [[extensions.targeting]]
  target = "cart.transform.run"
  input_query = "src/cart_transform_run.graphql"
  export = "cart-transform-run"

  [extensions.build]
  command = ""
  path = "dist/function.wasm"

query CartTransformRunInput {
  presentmentCurrencyRate
  cart {
    lines {
      id
      engraving: attribute(key: "Engraving") {
        value
      }
      cost {
        amountPerQuantity {
          amount
        }
      }
      merchandise {
        __typename
        ... on ProductVariant {
          product {
            title
          }
        }
      }
    }
  }
  cartTransform {
    metafield(namespace: "$app", key: "function-configuration") {
      jsonValue
    }
  }
}

// @ts-check

/**
 * @typedef {import("../generated/api").CartTransformRunInput} CartTransformRunInput
 * @typedef {import("../generated/api").CartTransformRunResult} CartTransformRunResult
 */

/**
 * Settings stored as JSON in the cart transform's metafield. The engraving
 * text comes from a line item property named "Engraving" on the product form
 * (<input name="properties[Engraving]">); change the attribute key in the
 * query if your theme uses another name.
 */
const DEFAULT_CONFIG = {
  // Added to each engraved unit, in the store's currency.
  fee: 10,
  // Appended to the product title on engraved lines.
  titleSuffix: " (engraved)",
};

/**
 * @param {CartTransformRunInput} input
 * @returns {CartTransformRunResult}
 */
export function cartTransformRun(input) {
  const config = { ...DEFAULT_CONFIG, ...input.cartTransform?.metafield?.jsonValue };
  // Prices in the input and output are in the customer's currency.
  const fee = config.fee * Number(input.presentmentCurrencyRate);

  const operations = input.cart.lines
    .filter((line) => line.merchandise.__typename === "ProductVariant" && (line.engraving?.value ?? "").trim() !== "")
    .map((line) => {
      const title = line.merchandise.__typename === "ProductVariant" ? line.merchandise.product.title : "";
      // A fixed unit price replaces the variant price, so it includes the fee.
      const unitPrice = Number(line.cost.amountPerQuantity.amount) + fee;
      return {
        lineUpdate: {
          cartLineId: line.id,
          title: `${title}${config.titleSuffix}`,
          price: { adjustment: { fixedPricePerUnit: { amount: unitPrice.toFixed(2) } } },
        },
      };
    });

  return { operations };
}